Principles of Milk Preservation. 93 



cream these clumps are broken down and the globules 

 are homogeneously distributed as in fig. 16. 



2. Cooked flavor. If milk is heated to 160 F., it ac- 

 quires a cooked taste that becomes more pronounced as 

 the temperature is further raised. The cause of this 

 change is not well known. Usually it has been explained ' 

 as being produced by changes in the nitrogenous ele- 

 ments in the milk, particularly in the albumen. Re- 

 cently , Thoerner 1 has pointed out the coincidence that 

 exists between the appearance of a cooked taste and 

 the loss of certain gases that are expelled by heating. 

 He finds that the milk heated in closed vessels from which 

 the gas cannot escape 



has a much less pro 

 nounced cooked flavoi 

 than if heated in an open 

 vessel. 



3. Fermentative 

 changes. Normally milk 

 undergoes the lactic acid 

 fermentation. If, 

 ever, it is heated to 

 temperature above 

 thermal death-point 

 these non-spore-bearing 

 organisms, as is the case 



., -i FIG. 16. Microscopic appearance of milk 



in pasteurizing, it does heated to 150 o F or above< The aggrega . 



not SOUr but CUrdleS by tionof ifat globules that is present in normal 



j, , milk is broken down, the globules being 



means Ot a rennet ter- homogeneously distributed throughout the 



ment Which is Secreted mil ^ serum. This lessens the consistency of 



, . , , , , , cream and makes it appear thinner. 



by those bacteria that 

 resist the heating process. 



4. Action toward rennet. The action of heat causes 

 the soluble salts of lime in the milk to be precipitated, and 



1 Thoerner, Chem. Ztg., 18: 845. 



